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I haven't, but it sounds like a great idea. The

only thing is if you measure the moisture percentage in the reservoir and it shows ok, what about the fluid in the lines and the caliper. That part might have more moisture in it. Either way, cool tool.

You do make a good point, I wonder if the fluid (m

in the reservoir would have more moisture in it than the fluid in the calipers and lines. The reservoir cap is vented and should let in more moisture than the seals around the caliper pistons. For $30 I may get one just to play with.

Actually, the brake system is totally sealed from

the atmospher. The vent hole in the top of the M/C cap is to allow the rubber seal in the lid expand as the fluid is moved out of the resevior into the brake system. If the system was left open to the atmospher, it would absorb moisture out of the air at an incredible rate and we would not have brakes for long due to corrosion. Unfortunately, moisture does find its' way into the system everytime you open the lid to check the level.Fritracer
Cincinnati, OH
2000 540ia Sport Package 9/99 Silver/Black
1970 Dodge Challenger T/A 420 stroker w/6 Pak 430 HP,
510 lbft torque

K-Bud..if U don't start driving the 540 I'm giving

"Drive It Like You Stole It, A Sickness for Quickness"The Bottle Rocket "King of Spray" 2 Stage Nitrous OxideDare to be different and stand out among the crowd of me-to-cars!No guts, no glory!Tire smoke, not traction control!Zionsville all aluminum radiator to replace the 3 leak prone Nissens Radiators!

None of my BMW's have such a rubber seal......

As I recall what you're describing was common on Ford and GM cars that had the cast iron reservoir built in to the MC. The corrugated rubber seal would follow the fluid level as it dropped as brakes wore. BMW reservoirs have to breathe to avoid creating a vacuum.

My cap does have an inner rubber washer, however(m

That doesn't block the 2 vent holes for the cap. The hole are visible on the outside of the cap under where the connector housing meets the side of the cap.
I found that Summit Racing also sells this brake fluid tester for $25.
I should have it in a few days. My fluid is ~2 years old and ready to be changed. I'll check what's in the resevoir and what comes out of the calipers to see if there is any real difference.

If you are changing fluids 1 to 2 X a year, Why

be concerned about it. I can see it's usefullness in possib;y testing ABS problems related to stuck components or maybe coroded pistons in the cALIPERS. I have only changed the Brake fluid 4 times in over 400,000 miles in the old VAN and only 2 times in the 540 and can never tel any difference.

In The Olden Days, we NEVER changed brake >>

fluid. The only time it was replaced was when one or more cylinders began leaking, probably due to not changing the fluid, and the system was rebuilt, requirirng bleeding out the air.
Ed in San Jose. BMW CCA member since 1987 (Nr. 62319). Golden Gate Chapter. '97 540i 6 speed. Build Date 3/97. Aspensilber over Aubergine leather.

+1

This is similar to running chemical tests on your motor oil to see if you are changing it often enough. Such tests satisfy intellectual curiosity, but once you look at the results, you may never feel the need to do it again.

Of course, if you have a caliper seize up, as we did recently at 195k miles on our '98 E36, you do get that guilty feeling that you haven't been flushing the brake system frequently enough.
'03 525i Sapphire Blu/Grey, SP, Nav
'02 325it Orient Blu./Grey, SP, Nav
'98 323is Arctic Sil./Blk, SP
'95 M3 Cosmos Blk./Blk